1. Field of the Invention
The invention relates to a process and apparatus for effecting separation of gases which are constituents from other low molecular weight gas mixtures by the controlled formation and decomposition of hydrates of the gases to be separated. The most widely used present technology for such separations is cryogenic distillation which is costly due to the low temperature required to liquefy methane. Other separation processes include gaseous diffusion using membranes and pressure swing adsorption, but these processes generally have not proven to be economical.
2. Description of Prior Art
The term "gas hydrates" has been applied for over a century to the solids that are formed by the combination of a number of gases with water. They constitute a class of solids in which small molecules occupy almost spherical holes in icelike lattices made up of hydrogen-bonded water molecules. Gas hydrates are structural combinations that remain associated not through strong attractive chemical forces but because strong mutual binding of the molecules of water makes possible the formation of cagelike structures that firmly enclose individual gas molecules. These combinations have been termed clathrates to distinguish them from hydrates in which chemical forces result in constant proportions of interacting species in combination with water molecules.
There is a considerable body of research devoted to equilibrium formation of hydrates stemming from the discovery over fifty years ago that blockages in natural gas pipelines exposed to low temperatures were due to formation of gas hydrate rather than of ice. Results of these studies have been summarized in a recent comprehensive treatise (E. Dendy Sloan, Jr., "Clathrate Hydrates of Natural Gases", Dekker N.Y. (1990)). A survey of the many studies of equilibrium properties of natural gas hydrates shows that correlations are adequate for prediction of the conditions that will result in hydrate formation; however, very little data are available on the composition of the hydrates formed. Hydrate compositions have been predicted by means of sophisticated statistical thermodynamic procedures for calculation of hydrate equilibria. Unfortunately, prediction by these procedures leads to conflicting results for the methane-nitrogen system. This problem stems from the unusual behavior of the nitrogen-water system.
Advanced methods of treating hydrate equilibria by statistical thermodynamics depend on microscopic properties of the structures. These crystal structures were studied in some detail in the 1950's by x-ray techniques. It was found that most gas hydrates constitute a class of solids in which small molecules of many types occupy almost spherical holes in ice-like lattices made up of hydrogen-bonded water molecules. The great majority of gas hydrates of pure substances have been shown to conform to either of two forms, designated as Structure I and Structure II respectively. Methane forms Structure I hydrates and until a few years ago it was thought that nitrogen also assumed this form of hydrate; however, it was recently discovered that nitrogen hydrate Conforms to Structure II (Davidson, D. W. et al. Mol. Cryst. Liq. Cryst., 141, 141 (1986)). It is believed that hydrates of gas mixtures will conform to either Structure I or Structure II, but statistical thermo-dynamic methods differ as to the methane-nitrogen proportions at which the transition occurs as the proportion of methane to nitrogen in a mixture of the two is increased. This is an important factor in determining the feasibility of separating high methane containing gases from such mixtures.